An important polycrystalline solar cell technology employs copper-compound semiconductors, such as copper indium diselenide, CuInSe.sub.2, called CIS, to convert solar energy into electricity or for use as a photocopy drum photosensitive surface. These CIS semiconductors are preferably bonded to a glass or alumina substrate. While CIS bonds acceptably to glass, direct CIS bonding to glass does not form a useful device because of the resistance of the CIS. Hence, some form of electrically conductive material is required on the back of the CIS semiconductor layer, which back layer in turn adheres to the substrate.
It is essential that back contact layers are inert with respect to the CIS semiconductor or at least not detrimental to the CIS if interdiffusion occurs. Group IB and IIIA metals such as Cu and Al diffuse into the layer and affect the CIS composition. Transition metals diffuse into the CIS layer during deposition. Even in very small quantities these are detrimental because they are understood to be deep traps in CIS which prevent photovoltaic behavior. Mo, the currently preferred back layer, has the advantage that it is almost completely inert with respect to Cu and In, and converts to Mo-selenides sufficiently slowly that the resulting presence of MoSe.sub.2 is not a major problem.
All of the commercial research and development work concerned with CIS solar cells is currently based on Mo back contacts. However, it is the inertness of Mo that allows adhesion failures, resulting in failure of the cells at the CIS/Mo interface. This failure is particularly common for In-rich CIS, whether slightly or more Indium-rich, which is the optimum composition for making photovoltaic devices as they currently have the best properties for conversion of sunlight to electricity. Mo is highly susceptible to accumulation of residual stresses during deposition as a thin film, which exacerbates delamination failure. Mo is also thought to result in a low-barrier Schottky contact to CIS which is not optimal.
Other back contact metals proposed to date in the art include aluminum, iron, nickel, chromium and pure copper. But these metals result in various chemical reactions and degradation of the CIS. That is, under the thermal deposition conditions those metals may chemically react with the CIS constituents, or diffuse into the relatively thin CIS layer. This results in degradation of the photo-electric conversion properties of the semiconductor.
Exemplary prior art showing a variety of approaches to back layer construction include MICKELSEN U.S. Pat. No. 4,392,451 (Boeing), which in FIG. 2 teaches a sputtered-on Mo base contact layer 32 on an alumina substrate 31 for a p-n type heterojunction solar cell which is vacuum deposited in a "composition-graded" manner. That is, there is a gradient in the overlying vacuum-deposited photovoltaic active materials starting with a low resistivity p-type semiconductor (shown in FIG. 4 as a Cu-enriched CIS), followed by a transient n-type semiconductor material (shown in FIG. 4 as a Cu-deficient CIS), together defining a transient p-n junction, followed by a low resistivity n-type semiconductor (CdS). The upper portion of the Cd/S is Indium-doped for superposition thereon of grid contacts 38 and an anti-reflective layer 40.
Regarding the base contact layer on the alumina substrate, MICKELSEN suggests in Col. 10, lines 9-12 that Mo/Au "could be used and, perhaps, other materials such as conventional nickel and graphite materials which have been commonly employed in conventional solar cells." Such multi-layer structures are commonly considered. However, Au has turned out to be unusable as a back contact constituent since it diffuses rapidly and damages photovoltaic performance if it enters the device layer, i.e., the semiconductor layer.
This MICKELSEN patent is old technology. In its day, over a decade ago, it may have been an improvement over the then-available CdS/CIS heterojunction solar cells. The focus is not on the base contact 32 (compare Prior Art FIG. 3 with the MICKELSEN invention FIG. 4). Rather, the idea was to form a gradient of the amount of Cu in the CIS layer from Cu-enriched adjacent the base contact Mo layer to Cu-deficient for contact with the CdS at the other surface, the top of which is In-doped. The interdiffusion between the discrete juxtaposed regions 35a, 35b of the CIS layer is said to define a transient p-n homojunction. However, the Cu-rich/In-rich bilayer device structure has been shown to be transient. SIMS analyses of such bilayers have shown that they consistently intermix during deposition leaving a uniform composition CIS layer behind. The Cu-rich material results from changes in the flux of atoms arriving from the vapor phase and not from the substrate metallization.
The MICKELSEN patent does not recognize or address the problems of delamination at the CIS/Mo interface.
A patent that does address the delamination problem is the 1990 POLLOCK et al. U.S. Pat. No. of ARCO, 4,915,745. Pollock et al. use a multi-layer structure consisting of a Mo back contact 12 on a glass substrate 10, with a Ga interlayer 13 between the CIS 24, 26 and the Mo back contact. The CIS is made by depositing a Cu layer 24 on the Ga, and then an In film 26, thereover. They are heated in the presence of Se to form CIS. The minimum thickness of Ga is 50 Angstroms. The resulting bond is said to prevent peeling, wrinkling and related problems. In Col. 5, lines 37-54, it states that Auger analysis was then performed to locate the various elements. Low concentrations (below detectable with Auger analysis) of Ga may be present in the CIS; Auger showed Ga only adjacent the Mo. Cu, In and Se had interdiffused to form a 2 micron CIS layer. Some Cu, In and Se were detected in the region "adjacent to the Mo" and "Se in a portion of the Mo film adjacent to the CIS film" (Col. 5, lines 45-48). However, there is clear evidence for Ga in the CIS layer from AES data, and a definite sensitization of the Mo layer results.
WIESMANN, in his 1985 U.S. Pat. No. 4,536,607, shows in FIG. 2 CIS 26 bonded to a glass substrate 24 via a 1/2 micron thick Mo interlayer 26 (Col. 8, 1.51-56), which acts as an electrical contact layer in an amorphous Si/CdS-Cu.sub.2 S heterojunction tandem photovoltaic cell. In FIG. 1, where 20 is CIS, 21 is Mo. This Wissmann patent teaches in Col. 6, lines 59-66 that the electrical contact layer 21 in FIG. 1 "may be a metal such as Mo, Ag, Au, Cu, Cr or the like, alone or in a mixture." In FIG. 1 the electrical contact layer is not a bonding layer. As to FIG. 2, Col. 7, lines 5-9, teach the metal layer 25 can be "a thin metallic layer" on an insulator 24 (see Col. 8, 1.51-56). There is no teaching of a mixture of Mo and Cu as a two-phase columnar layer for improved CIS-to-substrate bonding.
The remaining patents are of less pertinence: BASOL et al, in the 1987 International Standard Oil/BP Solar Ltd., U.S. Pat. No. 4,666,569 teaches a two-layer ohmic contact as a current collector (layers 14, 15 in FIG. 1) on a p-type semiconductor. Layer 13 is a CdTe semiconductor which is acid etched. Less than 50 Angstroms of Cu is deposited thereon to form layer 14. Layer 15 may be Mo, Ni or other metals, mixtures, and alloys (Col. 3, lines 52-55), but not Cu. Ni is preferred for contact layer 15, Cu for layer 14 with the Ni layer being greater than 1000 Angstroms thick. Col. 3, lines 60-65 indicates Mo cannot be substituted for the Cu layer 14. The Cu diffuses into the player (CdTe layer 13), Col. 4, lines 9-18, but Cu can not be used for layer 15 because then Cu will diffuse all the way through the CdTe layer and cause a short with the n-type CdS layer. Incidentally, InSn is used as the layer 11 to bond the CdS/Cd Te p-n layers 12, 13 to the glass substrate 10.
The BASOL et al., 1991 U.S. Pat. No. 5,028,274 of International Solar Electric Technology Inc. teaches using an intermediate layer of Te, Se, Sn or Pb between the surface of CIS 12 and a back layer 11 of Mo for bonding the CIS to the Mo. The Mo layer 11 is deposited on a glass substrate. Thus, this patent, like POLLOCK U.S. Pat. No. 4,915,745 teaches an intermediate layer, or modifying the Mo contact surface with another metal (Ga in Pollock, Te in Basol 274).
The MITCHELL 1987 U.S. Pat. No. 4,650,921 of Arco uses a tin oxide (SnO.sub.2) layer 14 on glass substrate 12 to bond to a CdTe semiconductor film. A Te-rich PbTe layer 18 is used to bond that CdTe film to a top, metal conductor layer 20, which may be Ni, Al, Au, Solder (Pb) and graphite-copper, with C-Ag being preferred.
The earlier 1984 MITCHELL U.S. Pat. No. 4,482,780 of D.O.E. shows use of an In or Sn layer 12 to bond CIS semiconductor layer 13 to glass. See Col. 5, lines 1 and 2. The CIS semiconductor is overlain with a CdS layer, Col. 6, lines 28-32.
The SZABO et al., 1978 Standard Oil U.S. Pat. No. 4,735,662 uses a 3-layer ohmic contact on top of a CdTe/Cds semiconductor (layers 9, 7 in FIG. 1). Cu is used as a 2 nm thick contact-forming layer 13 deposited on the CdTe. Then an isolation layer of C or Ni (layer 15) is used intermediate the Cu and the electrical conductor 19 connection layer 17, which is made of Cu covered by Ag, Al, or Ni, preferably Al. To bond the Cds to a glass substrate 3, an InSnO.sub.2 layer is used.
Accordingly, there is a need in the art to improve the mechanical and interlock between the back layer and semiconductor layer compositions while keeping excellent ohmic contact, in order to prevent photovoltaic cell failure due to delamination under operating conditions, thereby improving the useful life of solar cells and prevent electrical output drop-off due to back layer bonding failure.